Broadband radio frequency ferrite transformer providing close coupling

ABSTRACT

A broadband radio frequency transformer for use in the frequency range from 25 to 175 megahertz includes closely coupled windings on a ferrite rod providing low leakage inductance. A plurality of primary coils and a single continuous secondary coil are provided, with each primary coil bifilar wound with a portion of the secondary coil. The number of turns in the secondary coil is equal to the total number of turns in all the primary coils, or may be up to 50% greater than the total number of turns in the primary coils. The primary coils are connected in parallel to provide a step-up transformer having an impedance ratio related to the ratio of the number of turns in each primary coil to the number of turns in the secondary coil. The single coil can be used as the primary winding and the plurality of coils connected in parallel as the secondary winding for an impedance step-down transformer.

United States Patent 1191 A Fierstien et al.

[11] 3,721,932 l lMarch20, 1973 BROADBAND RADIO FREQUENCY FERRITETRANSFORMER PROVIDING CLOSE COUPLING Inventors: Gary N. Fierstien,Skokie; Paul H.

I Jacobs, Schaumburg, both of 111.

Assignee: Motorola, Inc., Franklin Park, 111.

Filed: Sept. 14,1971

Appl. No.: 180,515

Related US. Application Data Continuation of Ser. No. 15,619, March 2,1970,

abandoned.

U.S. Cl. ......336/65, 336/183, 336/205 Int. Cl "1101f 27/30 Field ofSearch ..336/180, 182,183, 170, 171,

References Cited UNITED STATES PATENTS 12/1927 Dowdell ..336/l83 X12/1950 'Thurston ..336/183 2/1961 Kajihara ..336/183 X PrimaryExaminerThomas .l. Kozma AztorneyMueller, Aichele & Rauner 5 7 ABSTRACTA broadband radio frequency transformer for use in the frequency rangefrom 25 to 175 megahertz in cludes closely coupled windings on a ferriterod providing low leakage inductance. A plurality of primary coils and asingle continuous secondary coil are provided, with each primary coilbifilar wound with a portion of the secondary coil. The number of turnsin the secondary coil is equal to the total number of turns in all theprimary coils, or may be up to 50% greater than the total number ofturns in the primary coils. The primary coils are connected in parallelto provide a step-up transformer having an. impedance ratio related tothe ratio of the number of turns in each primary coil to the number ofturns in the secondary coil. The single coil can be used as the primarywinding and the plurality of coils connected in parallel as thesecondary winding for an impedance step-down transformer.

2 Claims, 4 Drawing Figures PATENTH] MR 2 01975 Pie. 2

FIG.1

FIG. 4

lnvenfors GARY N. FIERSTIEN PAUL H. JACOBS QQMMQDJ ATT YS.

BROADBAND RADIO FREQUENCY'FERRITE TRANSFORMER PROVIDING CLOSE COUPLINGThis is a continuation of application Ser. No. 15,619, filed Mar. 2,1970, now abandoned.

BACKGROUND OF THE INVENTION Attempts have been made to provide aninterstage matching network for broadband semiconductor amplifiers.Since the semiconductor devices, such as transistors, have lowimpedance, a transformer winding coupled thereto must likewise have lowimpedance for proper impedance matching. This requires a transformerwith only a few turns which makes it difficult to provide closecoupling. In general the impedance of a transformer winding coupled to atransistor stage should be of the order of 2 to 6 ohms. Itmay be desiredto couple the amplifier stage to a 50 ohm line so that impedancetransformation from 4 to 50 ohms may be required.

Various matching networks such as- T and Pi networks have been proposedbut have been objectionable because theyhave had high Q. Strip lineshave also been proposed, but at frequencies of 25 to 175 megahertzrelatively long links are required. Prior transformers were notsatisfactory because of the broadband, high power and low lossrequirements and because of the range of impedances to be matched. Toprovide a broadband transformer it is necessary that the couplingcoefficient be high, closely approaching unity and this is difficult toobtain with a large turns ratio. Further it is desired that the Q bekept quite low, less than 2 in most cases.

SUMMARY OF THE INVENTION It is an object of the invention to provide aninterstage coupling network for a broadband solid state, high frequencyamplifier which provides close coupling and is of simple and inexpensiveconstruction.

Another object of the invention is to provide a broadband couplingtransformer for applications in the frequency range from 25 to 175megahertz, which has a high coupling coefficient and a low Q to providebroadband operation, and which provides impedance matching betweentransistor devices and a transmission line.

A further object of the invention is to provide a broadband radiofrequency transformer for use at high power, which provides the couplingcoefficient required without the use of a torroid core, to therebyreduce the cost and size of the transformer.

The transformer of the invention includes primary and secondary windingswound on a ferrite rod core. One of the windings is formed by aplurality of separate coils each having the same number of turns, andthe other winding is formed by a single coil having a number of turnswhich is either the same as the total number of turns in all theseparate coils, or is greater than this total number by no more than 50percent. For use as an impedance step-up transformer to couple atransistor amplifier to a 50 ohm line, the separate coils are connectedin parallel as the primary winding and the single coil forms thesecondary winding. Each separate coil is bifilar wound with a portion ofthe single secondary coil to provide a coefficient of coupling in therange from 0.97 to 0.98. By the use of three primary windings eachhaving one and one-half turns and a secondary winding having five turns,the transformer can be used for matching a push-pull transistoramplifier having an output impedance of the order of 6 ohms to an outputcircuit requiring an impedance of the order of 100 ohms. For use as animpedance step-down transformer, the single continuous coil can be usedas the primary winding, and the parallel connected separate coils can beused as the secondary winding.

BRIEF DESCRIPTION OF THE DRAWING 0 FIG. 1 shows the transformer of theinvention mounted on a printed circuit board;

FIG. 2 is an end view of the transformer on the board as shown in FIG.1;

FIG. 3 is a view of the transformer from the ends of the coils; and

FIG. 4 is a schematic diagram showing the connections of the transformerwindings.

DETAILED DESCRIPTION Referring now to the drawing, in FIG. I there is asecondary coil or winding 18 wound on the rod and having a plurality ofcoil portions each of which is bifilar wound with one of the primarycoils 14, '15 and 16. The physical arrangement of the windings isbelieved clear from FIGS. 1 and 3 wherein the turns of the primary coilsare shaded so that they can be distinguished from the secondary coil. InFIGS. 1 and 4 conductors numbered 1 and 8 are the ends of the secondarycoil, and conductors numbered 2-3, 4-5, and 6-7 are the ends of theprimary coils.

FIG. 4 is a schematic diagram of the transformer of the invention andshows the conductors numbered in the same manner as in FIG. 1. It willbe seen that each of the primary coils makes only one complete turnaround the core with the ends extending in the same direction tofacilitate insertion through openings in a printed circuit board. Eachprimary winding, therefore,

bifilar wound with the three primary coils 14, 15 and 16 as shown inFIGS. 1 and 3. This provides very close coupling between the primarycoils and portions of the secondary coil, providing a couplingcoefficient in the range of 0.97 and 0.98. It has been found that theturns ratio must be held small; in the range from 1:1 to 1:1.5,

in order to maintain this high coefficient of coupling.

This means that the number of turns in the secondary coil cannot besubstantially greater than the sum of the number of turns in all of theprimary coils.

In the example illustrated in FIG. 4, each primary coil has one andone-half turns so that the three coils have a total of four and one-halfturns. The secondary coil has five turns, so it has slightly more turnsthan the total number in all the primary coils. It is apparent thatone-half turn could be added to the secondary coil at each end thereofwhich would be closely coupled to the primary coils l4 and 16.Accordingly, the turns ratio could be somewhat increased without asignificant loss in the coefficient of coupling.

The number of turns in the coils of the primary winding must be keptlow. to provide the desired impedance has about one and a half turns.The secondary coil 18 is for matching to transistor amplifiers. In aradio transmitter using the transformers described, the impedance of theprimary winding of the transformer is of the order of 2 to 6 ohms andthe secondary impedance is of the order of 100 ohms. This permits theuse of two transformers at the outputs of the two push-pull amplifiersto connect the same in parallel to a 50 ohm transmission line.

The transformer is constructed to have very low leakage and low loss. Toaccomplish this the ferrite core is made of a nickel-zinc type ferritehaving low loss in the frequency range from 25 to 175 megahertz. Such aferrite is commercially available from the lndiana General Corporation,Keasbey, New Jersey, type Q-2 ferrite being suitable for use atfrequencies from 25 to 50 megahertz and type Q-3 ferrite being suitablefor use at frequencies from 50 to 200 megahertz. The windings are madeof heavy copper conductors having dual polyester coatings to provide thehigh power output required with minimum loss. As shown by FIGS. 1 and 3epoxy is applied to the end turns of the windings on the core to holdthe windings in position on the ferrite rod core. This is shown at 19and 20 in FIGS. 1 and 3.

Although a transformer has been described having a first winding withthree separate coils, transformers in accordance with the invention havebeen constructed with other numbers of coils, specifically with 2, 4 and5 coils. In these transformers the second winding has a singlecontinuous coil with portions bifilar wound with the separate coils ofthe first winding. The number of turns in the second winding can beslightly more than the total number of turns of the coils of the firstwinding, but to maintain close coupling can not be more than about 50percent greater. The transformer can be used as either a step-up or astep-down transformer by using the first winding as either the primarywinding or the secondary winding of the transformer.

The transformer described has'been found to provide effective interstagecoupling over a wide frequency band so that variable tuning is notrequired. The transformer provides impedance matching and at the sametime close coupling between the coupled stages.

We claim:

1. A broadband, closely coupled transformer for use on a printed circuitboard at frequencies in the range from 25 to megahertz including incombination, a ferrite rod core formed of nickel-zinc ferrite materialand having low loss, first and second winding means on said ferrite rodcore, means securing said winding means to said core, said first windingmeans including a plurality of separate coils wound on said rod core andpositioned axially therealong, said second winding means including asingle continuous coil wound on said rod core having a plurality of coilportions corresponding in number to the number of separate coils of saidfirst winding means, each of said coil portions of said second windingmeans being wound in a bifilar relation with one of said coils of saidfirst winding means, said continuous coil of said second winding meanshaving a number of turns in the range from the same number of turns asthe total number of turns in all said coils of said first winding meansto a number 50 percent greater than the total number of turns of allsaid coils of said first winding means, said plurality of coils of saidfirst winding means and sa|d continuous COll of said second windingmeans all having coil ends extending in the same direction for insertionthrough openings in the printed circuit board for mounting thetransformer on the printed circuit board, and conductor means on theprinted circuit board for connecting said coils of said first windingmeans in parallel.

2. A transformer in accordance with claim 1 wherein said first windingmeans is the primary winding of said transformer and includes threeseparate coils each having a number of turns in the rangefrom one to twoturns, and wherein said second winding means is the secondary winding ofthe transformer and has a number of turns in the range from three tofour times the number of turns in each of said coils of said firstwinding means.

1. A broadband, closely coupled transformer for use on a printed circuitboard at frequencies in the range from 25 to 175 megahertz including incombination, a ferrite rod core formed of nickel-zinc ferrite materialand having low loss, first and second winding means on said ferrite rodcore, means securing said winding means to said core, said first windingmeans including a plurality of separate coils wound on said rod core andpositioned axially therealong, said second winding means including asingle continuous coil wound on said rod core having a plurality of coilportions corresponding in number to the number of separate coils of saidfirst winding means, each of said coil portions of said second windingmeans being wound in a bifilar relation with one of said coils of saidfirst winding means, said continuous coil of said second winding meanshaving a number of turns in the range from the same number of turns asthe total number of turns in all said coils of said first winding meansto a number 50 percent greater than the total number of turns of allsaid coils of said first winding means, said plurality of coils of saidfirst winding means and said continuous coil of said second windingmeans all having coil ends extending in the same direction for insertionthrough openings in the printed circuit board for mounting thetransformer on the printed circuit board, and conductor means on theprinted circuit board for connecting said coils of said first windingmeans in parallel.
 2. A transformer in accordance with claim 1 whereinsaid first winding means is the primary winding of said transformer andincludes three separate coils each having a number of turns iN the rangefrom one to two turns, and wherein said second winding means is thesecondary winding of the transformer and has a number of turns in therange from three to four times the number of turns in each of said coilsof said first winding means.